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Re: basic-block and profile-based optimizing (was Re: New attribute"infrequent"?)
- To: Andreas Jaeger <aj at suse dot de>
- Subject: Re: basic-block and profile-based optimizing (was Re: New attribute"infrequent"?)
- From: Scott A Crosby <crosby at qwes dot math dot cmu dot edu>
- Date: Fri, 31 Aug 2001 13:25:08 -0400 (EDT)
- cc: David Edelsohn <dje at watson dot ibm dot com>, Jan Hubicka <jh at suse dot cz>, <gcc at gcc dot gnu dot org>, <pfk at fuchs dot offl dot uni-jena dot de>
On Fri, 31 Aug 2001, Andreas Jaeger wrote:
> David Edelsohn <email@example.com> writes:
> >>>>>> Andreas Jaeger writes:
> I forgot the following (Honza stated it in one of his first emails) -
> and this is IMO the key issue:
> If GCC notices that a function is infrequently used, e.g. only used for
> error handling, and normally not used, a call to this function can be
> placed out of line, a branch to a block that contains such a call can
> be predicted as not taken. But how does an application get the
> information that a function of the library is infrequently used?
This doesn't really matter, because the application itself presumabely
will be going through profile-based optimization.. Thus, when profiling
the application, we'll notice that the function call is seldom taken, and
thus we'll code it out-of-line at that time.
I don't think that adding such an attribute-mechanism to a library
interface is all that necessary, nor will it be worth the work and
In addition, this may be a pessimization.. If an infrequently called
library function is invoked from an application function, we only want to
code it out-of-line if the application function has a lot of invocations,
which we won't know till profile-based feedback. Otherwise, doing it out
of line could just gives size and complexity.
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